Directional drilling involves the drilling of a well bore along a deviated course in order to bottom out in a target region at a particular vertical and horizontal distance from the original surface location. Directional drilling is employed, for example, to obtain an appropriate well bore trajectory into an oil producing formation bed (or "pay zone") and then drill substantially within the pay zone. A horizontally drilled well can greatly increase the borehole volume in the pay zone with attendant increase in oil production. Recent advances in directional drilling equipment and techniques, some of which are referenced hereinbelow, have greatly improved the efficiency of certain drilling operations.
Before a directional drilling well plan is devised, the drilling team will usually have significant a priori knowledge of geological attributes of the local formations. This knowledge may be derived, for example, from survey and/or producing wells in the local area. Accordingly, in the directional drilling process, expected formation bed boundaries may be sought as markers from which to implement trajectory changes in the well bore. These boundaries are typically determined using borehole logging equipment; e.g. with so-called "wireline" logging equipment, wherein measurements are taken in a well bore (with the drill string removed) by lowering one or more logging devices in the well bore on a cable and taking measurements with the device(s) as the cable is withdrawn, and/or with so-called "logging-while-drilling" equipment wherein one or more logging devices are mounted on the drill string near the bit. Logging-while-drilling has obvious advantages for directional drilling in that trajectory changes, made in response to logging information, can be implemented without pulling the drill string.
Resistivity logging, which measures the electrical resistivity of formations surrounding an earth borehole, is a commonly used technique of formation evaluation. In so-called propagation logging, which is well suited to logging-while-drilling, energy is transmitted into the formations and propagates therein. Energy shed back into the borehole is measured at a receiver pair, in a so-called differential receiver arrangement, to obtain a phase shift measurement and an attenuation measurement. In one exemplary form of propagation logging, these measurements are sent to the surface of the earth where means are provided for determining, as a function of the phase shift, the resistivity (R.sub.ps) of formations at a relatively shallow depth of investigation around the receivers, and for determining, as a function of the attenuation, the resistivity (R.sub.a) of formations at a relatively deep depth of investigation around the receivers. Reference can be made to U.S. Pat. No. 4,899,112 for further description of equipment which utilizes propagating electromagnetic energy to obtain R.sub.ps and R.sub.a.
The resistivity logging values obtained from the described type of propagation logging device are useful in determining formation resistivity at two depths of investigation, and can also be used to locate bed boundaries under most conditions, such as by observing cross-overs between R.sub.ps and R.sub.a. However, when there is substantial invasion of the formations by the drilling fluid, bed boundary delineation can suffer. Also, there are situations which arise in directional drilling, particularly horizontal drilling, when improved bed boundary delineation may be desirable or necessary. Consider, for example, the situation where horizontal drilling has been initiated in a pay zone and it is desired to maintain a course in the pay zone and adjacent a boundary to the pay zone. As the drill path approaches the boundary at a very small angle (e.g., almost parallel to the boundary, which is a dip angle close to 90 degrees) it would be advantageous to know the boundary has been approached or reached as soon as possible (in terms of either time or position), and to be able to identify the location of the boundary with good accuracy. In such circumstance, prior boundary determination techniques may be inadequate, and the directional drilling operation can lose efficiency as steering corrections are missed or made at the wrong position, in the wrong direction, or later than they should be.
It is among the objects of the present invention to provide an improved method for directing or maintaining the drilling of a well bore with respect to formation bed boundaries.